1. Field Of The Invention
The present invention relates to a secondary electron multiplying apparatus, and more particularly, to a secondary electron multiplying apparatus having a high multiplying gain which is capable of preventing the ion feed back phenomenon.
2. Description Of The Related Art
Conventionally, there have been provided a secondary electron multiplying apparatus comprising plural separate dinodes for multiplying secondary electrons, and a continuous dinode type secondary electron multiplying apparatus comprising continuous multiplying channel for multiplying secondary electrons, which is also called a channel type secondary electron multiplying apparatus. The continuous dinode type secondary electron multiplying apparatus comprises a channel formed between plate-shaped channel bases arranged in parallel so as to oppose to each other or a pipe-shaped channel, and an emission surface for emitting secondary electrons having a high electrical resistance which is provided on an inner surface of the channel.
FIG. 1 shows a conventional continuous dinode type secondary electron multiplying apparatus 1.
Referring to FIG. 1, the continuous dinode type secondary electron multiplying apparatus 1 comprises secondary electron emission surfaces 4 and 5 of a high electrically resistive material capable of emitting secondary electrons, which are formed on respective opposing surfaces of plate-shaped channel bases 2 and 3 of glass or ceramics arranged so as to be parallel to each other, and then, there is formed a channel 6 having an entrance end 7 and an exit end 8 between the secondary electron emission surfaces 4 and 5.
A direct-current high voltage source 9 is electrically connected between the entrance end 7 and the exit end 8 of the channel 6 in order to apply an electric field for accelerating secondary electrons emitted from the secondary electron emission surfaces 4 and 5 in the axis direction of the channel 6. Further, there is arranged a collector electrode 11 for collecting secondary electrons multiplied within the channel 6 so as to oppose the exit end 8 of the channel 6. A direct-current voltage source 12 is electrically connected between the collector electrode 11 and the exit end 8 of the channel 6 in order to apply an electric field for leading the multiplied secondary electrons going out from the exit end 8 to the collector electrode 11.
In the secondary electron multiplying apparatus 1 constructed as described above, when primary electrons 13 are incident to the channel 6 from the side of the entrance end 7 thereof as indicated by solid lines in FIG. 1, the primary electrons 13 bombard the secondary electron emission surfaces 4 and 5, and then, secondary electrons are emitted therefrom. The emitted secondary electrons are accelerated by the above-mentioned electric field applied by the high voltage source 9, and further bombard the secondary electron emission surfaces 4 and 5, following a parabolic trajectory. Furthermore, additional secondary electrons are emitted therefrom. The above process is repeated so that the secondary electrons are multiplied in the channel 6, and the multiplied secondary electrons are collected onto the collector electrode 11.
In the channel 6 of the secondary electron multiplying apparatus 1, equipotential surfaces are formed so as to be perpendicular to the axis direction of the channel 6 as shown by dotted lines in FIG. 1. Therefore, the trajectory of the secondary electrons drawn in the channel 6 have parabolic shapes, and the number of the bombardment of the secondary electrons with the secondary electron emission surfaces 4 and 5 is in proportion to the ratio of the distance in the axis direction of the channel 6 to the distance between the channel bases 2 and 3. Accordingly, in order to heighten the gain of the secondary electron multiplying apparatus 1, it is necessary to increase the above-mentioned ratio. Further, it is necessary to form the equipotential surfaces in parallel so as to be perpendicular to the axis direction of the channel 6, and to further increase the length of the axis thereof.
In order to solve the above-mentioned problems of the conventional continuous dinode type secondary electron multiplying apparatus 1, there was proposed an inclined electric field type secondary electron multiplying apparatus capable of obtaining a high multiplying gain at a low acceleration voltage by utilizing an inclined electric field for accelerating secondary electrons in the Japanese patent examined publication (JP-B2) No. 50-16145/1975, the Japanese patent examined publication (JP-B2) No. 50-25303/1975, the Japanese patent examined publication (JP-B2) No. 52-38378/1977, and the U.S. Pat. No. 3,235,765 etc..
FIGS. 2 and 3 show secondary electron multiplying apparatuses 24 and 25 disclosed in the Japanese patent examined publication (JP-B2) No. 50-25303/1975.
Referring to FIGS. 2 and 3, in the secondary electron multiplying apparatuses 24 and 25, there are arranged a first plate 21 comprising an inner surface capable of emitting secondary electrons and a second plate 22 of a high electrically resistive material or an electrically conductive material to form a channel 23 having an entrance end 23a and an exit end 23b so that the interval between the first and second plates 21 and 22 is enlarged as approaching the exit end 23b of the channel 23. Then, equipotential surfaces formed between the first and second plates 21 and 22 which are indicated by dotted lines in FIGS. 2 and 3 cross the first plate 21 at an acute angle, and the potentials of the equipotential surfaces become higher as approaching the exit end 23b. Therefore, the secondary electrons bombard a secondary electron multiplying surface 26 of the first plate 21, drawing loci as indicated by real lines in FIGS. 2 and 3, and are multiplied. Finally, the multiplied secondary electrons are collected onto a collector electrode 27.
In the above-mentioned conventional secondary electron multiplying apparatuses 24 and 25, the interval between the first and second plates 21 and 22 is narrowed on the side of the entrance end 23a of the channel 23, and is enlarged approaching the exit end 23b thereof. Therefore, in order to make charged particles be incident into the channel 23 from the side of the entrance end 23a thereof, it is necessary to taper down a beam of charged particles to be incident thereto as soon as possible.
A further problem is that in a general secondary electron multiplying apparatus, since the density of the electrons is high at the exit end 23b of the channel 23 because of multiplying the secondary electrons, the electrons collide with a remaining gas, and the remaining gas is ionized so as to become ions having a polarity opposite to that of the electrons. In the above-mentioned conventional secondary electron multiplying apparatuses 24 and 25, since the interval between the first and second plates 21 and 22 is enlarged on the side of the exit end 23b of the channel 3, there is caused the problem of the so-called ion feed back phenomenon wherein the above-mentioned ions going out from the channel 23 are returned to the channel 23, and bombard the second plate 22 so as to emit secondary electrons.
Further, there has been proposed a disk like shape type secondary electron multiplying apparatus capable of multiplying secondary electrons when the secondary electrons extend radially outward from the center thereof toward the outer edge thereof.
FIG. 4a is a schematic top plan view showing a structure of the conventional disk like shape type secondary electron multiplying apparatus 51 disclosed in U.S. Pat. No. 3,436,590, and FIG. 4b is a schematic cross sectional view taken on a line IVb- IVb' of FIG. 4a of the disk like shape type secondary electron multiplying apparatus 51.
Referring to FIGS. 4a and 4b, the disk like shape type secondary electron multiplying apparatus 51 comprises two circular plates 52 and 53. Grooves 54a to 54e are formed in a shape of concentric circles on the inner surface of the circular plate 52 so that each groove shown in the cross section of FIG. 4b has a shape of a circular arc and the radius of curvature thereof is gradually enlarged away from the center of the circular plate 52. On the inner surfaces of respective grooves 54a to 54d except for the outermost groove 54e, there is formed an electrically conductive layer 71 and a secondary electron emissive layer 55 of a material having a relatively large secondary electron emission coefficient. Further, a collector electrode 56 is formed on the inner surface of the outermost groove 54e so as to be electrically connected to the conductive layer 71.
On the other hand, there is formed a concave 57 in the circular plate 53 on the side of the inner surface of the circular plate 53 which opposes the inner surface of the circular plate 52, and grooves 58a to 58e are formed at respective positions shifted by half a groove width from those of respective grooves 54a to 54e in the radial direction on the surface of the concave 57 which opposes the inner surface of the circular plate 52. Further, there are formed an electrically conductive layer 72 and a secondary electron emissive layer 59 of a material having a relatively large secondary electron emission coefficient on the inner surfaces of respective grooves 58a to 58e.
The above-mentioned two circular plates 52 and 53 are overlapped and fixed integrally on each other so that the inner surfaces thereof oppose each other, so that a channel 70 is formed between the circular plates 52 and 53. A direct-current high voltage source 63 is electrically connected between a terminal 61 provided in the center of the circular plate 52 which is electrically connected to the secondary electron emissive layer 55 through the conductive layer 71, and a high voltage terminal 62 which is electrically connected to the secondary electron emissive layer 55 formed on the groove 54d through the conductive layer 71. On the other hand, a direct-current high voltage source 66 is electrically connected between a terminal 64 provided in the center of the circular plate 53 which is electrically connected to the secondary electron emissive layer 59 through the conductive layer 72, and a high voltage terminal 65 which is electrically connected to the secondary electron emissive layer 59 formed on the groove 58e through the conductive layer 72. Thus, there is applied an accelerating electric field in the radial direction, namely, in the direction from the center of each of the circular plates 52 and 53 to the outer edge thereof. Furthermore, a direct-current voltage source 68 is connected between the above-mentioned high voltage terminal 65 and a collector terminal 67 which is electrically connected to the collector electrode 56 through the collector terminal 67. A coupling capacitor Cc is also connected to the collector terminal 67.
When charged particles are incident into the channel 70 from an entrance hole 69 formed in the center of the circular plate 52, they bombard the secondary electron emissive layer 59 formed on the circular plate 53, and then, secondary electrons are emitted therefrom. Thereafter, the emitted secondary electrons are multiplied bombarding the secondary electron emissive layers 55 and 59 by an electric force of the above-mentioned accelerating electric field, and finally, the multiplied secondary electrons are taken out as a secondary electron current from the collector electrode 56.
In the above-mentioned disk like shape type secondary electron multiplying apparatus 51, a problem is that it is difficult to form the grooves 54a to 54e and 58a to 58e respectively having the above-mentioned particular shapes and also to form the secondary electron emissive layers 55 and 59 with a high precision. Therefore, the disk like shape type secondary electron multiplying apparatus has not yet been put into practical use, actually.